It is known to remove oxides of nitrogen (NO.sub.x) pollutants from exhaust streams or other gaseous emission sources which contain excess oxygen by adding NH.sub.3 to the gas and passing the resulting mixture over a catalyst so that the NH.sub.3 and NO.sub.x react to form non-polluting N.sub.2 and H.sub.2 O.
A wide variety of catalysts has been proposed for this purpose. One such catalyst is iron sulfate which has been described as an SCR catalyst in U.S. Pat. No. 4,107,272. However, study indicates that when iron sulfate is used as an SCR catalyst, the sulfate is really only stable in gases containing significant amounts of sulfur oxides (SO.sub.x) and it appears to be rapidly deactivated at typical SCR temperatures such as 400.degree. C., if the gas being treated, e.g. flue gas, only has at most a small amount (e.g. up to 200 ppm) of SO.sub.x.
Thus, in the absence of any significant amounts of SO.sub.x, iron sulfate is not stable at temperatures higher than approximately 350.degree. C. and is decomposed to iron oxide, the SCR activity of the sulfate decreasing as the iron sulfate is decomposed to the oxide.
Natural gas is a common fuel, particularly in the United States, and flue gases generated from the combustion of natural gas are free of SO.sub.x. As a consequence, iron sulfate cannot be utilized commercially as an SCR catalyst for such SO.sub.x -free flue gases because of the rapid catalyst deactivation which takes place.
An ideal SCR catalyst is one which in practice has a high level of activity which is maintained over long periods of use whether or not SO.sub.x is present in the gas being treated. The principal purpose of the present invention is to provide such a catalyst.